On my Plus 8 I put a couple of set screws (from above) in the fixing holes, and a nut (with shakeproof washer) on the underside, so I in effect I had two threaded studs to secure the ECU to, just have to get underneath and apply a washer and nut to each stud to secure the ECU. Otherwise its a bit of a game, as your arms aren't long enough to be above and below the firewall.
A suggestion, try and make sure the case of the ECU is not grounded via the mounting bolts. The ECU picks up a ground at the engine end of the harness. If you should have an issue with the grounding strap on the engine, and the ecu case is grounded, the starter current will go up the harness, through that connector, and cook the ECU circuit board. I used some grommets fitted into the mounting tabs on the ECU, so the mounting bolts pass through the grommets.
For those who haven't got under to look at the ECU, the ECU is held up by a bracket with a felt liner, so the ECU can be slipped into the bracket, that takes the weight of the ECU, and then it is secured with a couple of bolts that stop it from slipping out. (or that is how it is fitted on my Plus 8)
